1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride polymer, comprising polymerizing vinyl chloride or a monomer mixture comprised of mainly vinyl chloride in a polymerization vessel fitted with a reflux condenser, and in particular to a process for producing a vinyl chloride polymer improved in the composition of a reaction system and the addition timing for additives in order to prevent foaming when reaction heat is removed by means of the reflux condenser or when unreacted monomers are recovered.
2. Description of the Prior Art
In recent years, in the production processes for a vinyl chloride polymer, for the purpose of improving the production efficiency, it is now attempted to enlarge the size of a polymerization vessel and to reduce polymerization time. As a method of reducing the polymerization time, it is performed to fit a polymerization vessel with a reflux condenser to improve heat removal efficiency in attempt to reduce time. As another method it is performed to increase recovery rate for unreacted monomers after completion of polymerization in attempt to reduce the time for the monomer recovery.
However, increase of heat removed by the reflux condenser or increase of the unreacted monomer recovery rate results in increase of degree of vacuum at the gas phase section in a polymerization vessel, so that vaporization of vinyl chloride monomer or the like dissolved in a slurry is accelerated. Consequently, foam generation is increased, the foam is accumulated on the surface of the liquid, and then the foam rises, thereby causing a phenomenon (carry-over) wherein polymer particles reach the upper part of the polymerization vessel. In a serious instance, the carry-over will reach the inside of a reflux condenser and around the inlet of an unreacted monomer recovery pipe provided for a polymerization vessel, resulting in that the polymer particles are deposited as scale not only on the upper part of the polymerization vessel but also inside a reflux condenser and around the inlet (recovery port) of the unreacted monomer recovery pipe. The scale deposition like this not only causes a big impediment to a production process such as maintenance after polymerization, but also adversely affects the quality of polymer products, such as causing fish eyes (FE) in the resulting polymeric products, if the subsequent polymerization is carried out with use of the polymerization as it is.
As described above there have been quite limitations on the use of a reflux condenser, it cannot be said that the functions of a reflux condenser are sufficiently utilized.
Heretofore, for the purpose breaking foam at the gas phase section in a polymerization vessel, there are proposed a method of conducting polymerization while stirring the gas phase section by means of rotating blades (see Japanese Post-examination Patent Publication (kokoku) 60-42804), and a method of spraying water or a foam-controlling agent over foam using spray nozzles such as flat nozzle, a full-cone nozzle, or the like (Japanese Post-examination Patent Publication (kokoku) 50-30106).
These methods bring about a certain level of anti-foaming effects; however, the effects are not sufficient. Therefore, it is expected that a further improved anti-foaming technology is developed.
As one of the methods is known a method in which an anti-foaming agent is added during polymerization. However, the use of the anti-foaming agent only provides insufficient foam-breaking effects. The increase of the quantity of the anti-foaming agent to enhance the foam-breaking effects, results in a problem of scale deposition, and therefore the quantity of the anti-foaming agent is naturally limited.
It is an object of the present invention to provide a process for producing a vinyl chloride polymer which can certainly prevent the polymer slurry from foaming, rising, and scattering during polymerization or recovery of unreacted monomers, and can make a reflux condenser exhibit its heat-removing effects fully, and can recover unreacted monomers efficiently.
A process for producing a vinyl chloride polymer according to the present invention, comprises polymerizing vinyl chloride or a monomer mixture comprised of mainly vinyl chloride in an aqueous medium in a polymerization vessel fitted with a reflux condenser, said process comprising:
starting to add
(A) an anti-foam agent, and
(B) a partially saponified polyvinyl alcohol with an average degree of polymerization of 100 to 500 and a saponification degree of 75 to 85 mole % to a polymerization system during the operation of said reflux condenser, and
continuing the addition of (A) and (B) stated above until the completion of recovery of unreacted monomers from said polymerization vessel.